1. Technical Field
The present invention relates to a physical quantity sensor and an electronic apparatus including the physical quantity sensor.
2. Related Art
In related art, as a physical quantity sensor, an electrostatic capacitance sensor in which a structure having a movable electrode supported by a fixed part via an elastic element such as a torsion spring is formed, the movable electrode moves close to or away from a fixed electrode in response to an acting external force or the like, and thereby, various physical quantities such as acceleration, angular velocities, or the like may be detected by detecting the change in electrostatic capacitance between the electrodes has been known.
As the electrostatic capacitance sensor, an ultracompact mechanical acceleration sensor adapted to detect a physical quantity in a vertical axis direction by a swing stage (mass part) swingably supported by a torsion rod (torsion spring) in a hollow space between two semiconductor wafers and swinging (displaced) like a seesaw due to an applied physical quantity such as acceleration has been disclosed (for example, see Patent Document 1 (JP-A-9-189716)).
The ultracompact mechanical acceleration sensor (hereinafter, referred to as “acceleration sensor”) in Patent Document 1 is the electrostatic capacitance sensor. Thereby, in the acceleration sensor, from the following general expression (1) of the electrostatic capacitance, in order to increase detection sensitivity, for example, the swing stage as a movable electrode and a first electrode as a fixed electrode opposed to the swing stage are made closer to increase the electrostatic capacitance so that the electrostatic capacitance may change to an appreciable extent for small displacement of the swing stage and small acceleration may be detected.C=εS/d  (1)(an electrostatic capacitance is C, an area of an opposed electrode is S, a distance between the opposed electrodes is d, permittivity is ε)
However, in the acceleration sensor, a surface of a first semiconductor wafer with the first electrode (fixed electrode) formed thereon is flat and the swing stage is formed like a flat plate.
Accordingly, in the acceleration sensor, for example, when the swing stage is displaced due to inertia force of the applied acceleration, the displacement may be suppressed by fluid resistance (squeeze film damping) of a gas existing between the swing stage and the surface of the first semiconductor wafer, and response may be slower and a detection range may be narrower.
Further, in the acceleration sensor, the swing stage may be stuck to the surface of the first semiconductor wafer due to charging caused by static electricity.